Selective catalytic reduction (SCR) is commonly used to remove NOx (i.e., oxides of nitrogen) from the exhaust gas produced by internal engines, such as diesel or other lean burn (gasoline) engines. In such systems, NOx is continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NOx.
Ammonia is often used as the reductant in SCR systems. The ammonia is introduced into the exhaust gas by controlled injection either of gaseous ammonia, aqueous ammonia or indirectly as urea dissolved in water. The SCR catalyst, which is positioned in the exhaust gas stream, causes a reaction between NOx present in the exhaust gas and a NO reducing agent (e.g., ammonia) to convert the NOx into nitrogen and water.
Proper operation of the SCR system involves precise control of the amount (i.e., dosing level) of ammonia (or other reductant) that is injected into the exhaust gas stream. Injection of too much reductant causes a slip of ammonia in the exhaust gas, whereas injection of a too little reductant causes a less than optimal conversion of NOx. Thus, SCR systems often utilize NOx sensors in order to determine proper reactant dosing levels. For example, a NOx sensor can be positioned in the exhaust stream between the engine and the SCR catalyst for detecting the level of NOx that is being emitted from the engine. This is commonly referred to as an engine out NOx sensor or an upstream NOx sensor. An electronic control unit (ECU) can use the output from the engine out NOx sensor (and/or other sensed parameters) to determine the amount of reductant that should to be injected into the exhaust stream.
For example, the accuracy of NOx sensors can be affected by environmental and/or operating conditions such as dew point, system voltage, oxygen concentration, and the like. For example, some NOx only work properly when the exhaust gas is above a threshold temperature which can be on the order of 125-130° C. As a result, such sensors may not suitable for determining dosing levels during certain engine operating conditions, such as low idle or engine warm-up. Hence, it is desirable to provide an alternative method for determining the NOx level in an engine's exhaust, particularly during conditions when a NOx sensor is prone to producing inaccurate readings. It may also desirable to be able to switch between control based on the NOx sensor and/or the alternative NOx determination method based on operational and/or environmental conditions.